1. Field of the Invention
The present invention relates to a proximity detector utilized to sense the presence of a piston reciprocatively mounted within a cylinder bore. More particularly, the invention relates to a device and method for providing such a proximity detector which is generally noninvasive of the cylinder housing assembly.
2. Description of the Prior Art
In the operation and control of fluid actuated cylinders, proximity detector devices are often utilized to indicate presence of the moving piston therein. Frequently, such devices are employed to detect the piston at respective terminal limits of its reciprocative stroke. In a manufacturing environment, for example, this information can be advantageously used to maintain efficient workpiece flow.
Prior art proximity detector devices have generally required modification of the cylinder housing assembly. For example, some commercially available proximity detectors utilize a magnetic detector mounted externally of the cylinder assembly to detect a magnetic member attached to the piston. Passage of the magnetic member causes activation of the detector, thus affirmatively indicating presence of the piston. Generally, however, such devices are limited to applications in which the cylinder is constructed of a nonmagnetic material such as aluminum. Hydraulically actuated cylinders are typically made of steel, which inhibits the external detection of a magnetic field. Other proximity detector devices, equally useable with pneumatically and hydraulically driven cylinders, require an even greater degree of invasive modification to the cylinder assembly.
The phenomena of ultrasonic energy propagation has been utilized in a number of applications. For example, nondestructive testing (NDT) techniques often utilize ultrasonic pulses to detect flaws in metal and weld joints. Ultrasonic energy propagation has also been used to measure the flow of fluid in a conduit. Furthermore, various devices have been constructed wherein distance to an object or thickness of a workpiece is measured by the duration between transmission of an ultrasonic pulse and reception of the subsequent echo.
One such device is used to measure the position of a piston moving within a hydraulic cylinder. The ultrasonic transducer in this device is invasively mounted to the end cap of the cylinder. An ultrasonic pulse is transmitted into the hydraulic fluid coaxial to movement of the piston. An echo is reflected from the face of the piston and returns to the transducer after a duration proportional to the distance from the transducer.